Exocytosis of neurotransmitters, hormones and other cell secretory products is generally believed to occur by fusion of secretory vesicles with the plasma membrane and subsequent breakage of the membrane separating vesicle interior from extracellular milieu. The mechanism of exocytosis has been investigated in adrenal medulla, platelets and other tissues. We have discovered a new protein in adrenal medulla, "synexin", that fuses secretory vesicle membranes to one another if calcium is also present. This protein may be the intracellular receptor for calcium. We have also discovered that the motive force for the release event may be an osmotic rupture of the secretory vesicle in the vesicle-plasma membrane complex and have discovered drugs that block this process. On this basis we have proposed a chemiosmotic hypothesis for exocytosis.